Microwave diode mixers are well known in the art. In a down converter, the mixer has an input port for receiving an RF signal and an input port for receiving a local oscillator (LO) signal. The mixer has an output port delivering a plurality of frequencies, including an IF signal at a frequency which is the difference between the RF and LO signal frequencies. The frequencies which are output from the mixer are the modulation products which exist according to the heterodyne principle by which the mixer operates, wherein an RF signal and an LO signal are applied to a nonlinear element such as a diode.
In an up converter, the mixer has an input port for receiving an LO signal and an input port for receiving an IF signal. The mixer has an output port for delivering a plurality of frequencies, including RF signals which are the sum and difference of the LO and IF signal frequencies. The frequencies which are output from an up converter mixer are likewise the modulation products which exist according to the heterodyne principle by which the mixer operates.
A double balanced diode mixer has two pairs of diodes either cross connected (lattice modulator) or connected in a loop (ring modulator), which are equivalent. The individual diodes in each pair are commonly poled, and the composite pairs may be commonly or oppositely poled.
In addition to the IF signal, a down converter mixer generates a signal which is the image of the RF signal but on the opposite frequency side of the LO signal AS an example, a mixer receiving a 6 GHz (6,000 MHz) input RF signal and a 5,930 MHz input LO signal would generate a 70 MHz IF signal and a 5,860 MHz image RF signal. About half of the incoming RF power is used in generating this image frequency signal. The loss in converting an incoming RF signal to an IF signal is thus significantly increased by this image frequency generation. Likewise for up conversion, conversion loss is caused by frequency generation of an unused sideband. For further background regarding mixer operation, reference is made to my article entitled "Trace Phase States to Check Mixer Designs", Microwaves, June 1980, pages 52-60.
Prior art devices have eliminated the image frequency signal in a variety of ways. Some devices use filters to prevent the image frequency signal from entering the input signal circuitry, but this still results in an energy loss to the system, and also reduced bandwidth. Some devices provide an open or short circuit at the diodes in order to recover, or reduce the loss of, the image frequency energy. U.S. Pat. Nos. 2,834,876 and 3,681,697 show image recovery mixers where the image frequency power is reflected back to the mixer to provide recovery of the image frequency energy. These devices are discussed in U.S. Pat. No. 3,831,097 to Neuf, which shows an image recovery mixer system having two double balanced mixers each of the diode bridge type, one set of opposing diagonals of one bridge being interconnected with one set of opposing diagonals of the other bridge to cancel the image frequency signal directly between the diode mixers.
Another patent to Neuf, U.S. Pat. No. 3,652,941, shows a mixer with a single diode-quad bridge. Each side of the bridge is input from balanced lines referenced to each other, i.e. balanced circuits that don't have a ground reference at the diode terminals are used for both the RF and LO inputs to the mixer. One RF line is on top of a dielectric substrate and the other RF line is on the bottom. The balanced LO lines are likewise juxtaposed on opposite sides of the substrate.
Other mixer circuits with a single diode-quad mixer bridge are shown in a paper entitled "A New `Phase-Typed` Image Enhanced Mixer", by L. E. Dickens and D. W. Maki, 1975 IEEE MTT Symposium, pp. 149-151, and in Ernst et al., U.S. Pat. No. 3,772,599. The mixer is formed by two pairs of diodes providing a bridge across a slot transmission line. While these mixers have proved useful for their intended purpose, they suffer the inherent disadvantages and undesirable characteristics of slot line circuits, including the various constraints on the dimensions of the slot, minimum ground plane spacing on either side of the slot, the requisite high dielectric constant substrate, transmission mode waveguide problems, etc. The width of the slot should usually be no greater than 5 to 10 mils. A dielectric substrate having a high magnitude of relative dielectric constant of 9 to 10 or greater is needed. If the slot line is to be generally useful as a transmission line, the fields must be closely confined to the slot. Close confinement can be achieved with slots of realistic dimensions by using a fairly high dielectric constant substrate. If the guide wavelength is roughly 30 to 40 percent of the free space wavelength, the fields will be adequately confined. Low dielectric constant substrates, for example less than about 8, are typically not suitable for slot line application because the energy is not well confined to the slot. A further drawback is that specialized and expensive fabrication techniques are required, such as a thin film operation depositing gold on ceramic.